1. Field of the Invention
This invention relates to a continuously variable power lens system for use as a copying lens, a photoengraving lens, a relay lens in an optical system, or the like which is generally used at magnifications of the image near unit magnification and whose power can be varied with the finite conjugate distance fixed.
2. Description of the Prior Art
Recently, there is an increased demand for a copying system or a photoengraving system in which the magnification or reduction ratio of the image can be continuously varied. In conventional systems of this type the magnification or the reduction ratio of the image can be varied only to a plurality of discrete values.
As is well known, in order to vary the magnification (In this specification, the term "magnification" should be broadly interpreted to include magnification not larger than unit magnification.) using a fixed focal length lens system, at least two of the object plane, the image plane and the lens system must be moved. However, it is undesirable to move the surface of the object plane or the image plane from the viewpoint of operational convenience and because a complicated link mechanism is required. Therefore, the magnification has generally been varied without moving either the object plane or the image plane by inserting a plurality of mirrors into the object space between the object plane and the lens system or the image space between the image plane and the lens system and moving the mirrors to compensate for change of the conjugate distance. However, this method is disadvantageous in continuously varying the magnification since an expensive servomechanism is required to relate the movement of the mirrors with the magnification, and because a mirror transfer mechanism becomes necessary for moving the mirrors, which are relatively large, whereby the overall size of the system is significantly enlarged.
The magnification can also be varied without moving either the object plane or the image plane by changing the focal length by moving only the lens system and adding an attachment lens. However, by this method, there can only be obtained a magnification which is determined by the characteristics of the main lens system and the attachment lens added, and the magnification cannot be continuously varied.
On the other hand, attempts have been made to develop a zoom lens system whose magnification is continuously variable without changing the conjugate distance by moving the whole lens system and some of its lens elements. (See Japanese Unexamined Patent Publication No. 56(1981)-159614, and Japanese Unexamined Patent Publication No. 57(1982)-67909, for example.) Either of the zoom lens systems is deemed to comprise an ortho-metha-type main lens system and an attachment lens system disposed in front and the rear of the main lens system. The zoom lens system disclosed in the first-mentioned Japanese Unexamined Patent Publication is of the outer element movable type, and the main lens system is stationary while the attachment lens system is movable. On the other hand, the zoom lens system disclosed in the latter Japanese Unexamined Patent Publication is of the inner element movable type, and the main lens system is movable while the attachment lens system is stationary. In both zoom lens systems, the power of the attachment lens system is negative, and both systems consists of four groups, the powers of which are negative, positive, positive and negative, respectively. It is well known that the zoom lens system can be miniaturized by the negative-positive-positive-negative arrangement. However, the size of the obtained zoom lens system and the amount of movement of the movable lens groups depend upon distribution of the powers of the negative and positive lens groups.
In either of the zoom lens systems disclosed in the above identified Japanese Unexamined Patent Publications, if the power of the negative lens groups is too large, the performance of the positive lens groups is adversely affected, since the negative lens groups function like an attachment lens. Therefore, the power of the negative lens groups must be small in the zoom lens systems, which inherently results in a large amount of movement of the movable lens groups. This, in addition to the increased number of lenses, prevents the zoom lens systems disclosed in the above Japanese Unexamined Patent Publications from adequately meeting the demand for a compact zoom lens system. Further, zoom lens systems like that disclosed in the Japanese Unexamined Patent Publication No. 56(1981)-159614, in which the outer lens groups are movable to change the length of the overall lens system and the amount of movement of the movable lens groups is large, are disadvantageous in that greater mechanical limitation must be imposed to prevent interference of the movable lens groups with other parts in the system in which the zoom lens system is employed.
Use of the zoom lens is advantageous, on the one hand, in that the mechanism for changing the magnification of the image is highly simplified, but is disadvantageous, on the other hand, in that the conjugate distance must be long, the field angle must be small in order to obtain performance equivalent to that of fixed focal length lens systems, and the number of lenses is increased, whereby it becomes difficult to apply the zoom lens to inexpensive copying systems and the like.
Generally, in a variable focal length lens system whose focal length is varied without changing the conjugate distance, the entire lens system and two of the lens groups constituting it must be moved.
In photographic zoom lens systems which are designed to operate at front focal distances near theoretical infinity, the image plane can be maintained at a fixed position by changing the position of the principal point without moving the entire lens system. However, in a copying lens, for example, which is used at magnifications near 1x and in which the conjugate distance is finite, the conjugate distance varies by a large amount when the magnification changes and the object plane and the image plane are fixed at a constant position mostly by changing the magnification through movement of the entire lens system and partly by changing the magnification by slightly changing the focal length through axial movement of some of the elements.
In copying systems and photoengraving systems, in which the magnification is generally varied within a range including unit magnification (1x), it is advantageous, from the viewpoint of the distortion, the chromatic aberration of magnification, coma and the like, to arrange the lenses in a symmetrical manner and to change the magnification by moving the movable lenses without breaking the symmetry. That is, it is preferable to move the movable groups disposed on opposite sides of the stop in a symmetrical manner maintaining the symmetry in response to the movement of the entire lens system so that the symmetry in the entire lens system is not significantly broken and the conjugate distance can remain constant.